The present invention relates to a recording and/or reproducing optical system and an objective lens for an optical information recording medium, and more specifically to a recording and/or reproducing optical system and an objective lens for an optical information recording medium by which light flux from a laser source is converged on an information recording surface of the optical information recording medium as an optical spot through a transparent substrate, and information is recorded on the information recording surface and/or information on the information recording surface is reproduced.
Conventionally, various types of optical systems of optical pick-up apparatus, by which information on a CD (compact disk) as an optical information recording medium is recorded and/or reproduced, are widely known. Further, recently, development of a DVD (digital video disk) which is an optical disk having the same size as that of the conventional CD, and has larger capacity than the conventional CD, is being advanced. To be more precisely, in the DVD, as the a short wavelength red semiconductor laser has been put into practical use, this laser is used as the semiconductor laser to be used corresponding to an increase of the capacity; and the density of the DVD is highly increased such that a numerical aperture NA of an objective lens of the optical pick-up is 0.6, and the thickness of the transparent substrate is 0.6 mm which is a half of the thickness of the conventional CD, and further, a track pitch is 0.74 .mu.m and the minimum pit length is 0.4 .mu.m, which are smaller than a half of the track pitch of 1.6 .mu.m and the minimum pit length of 0.83 .mu.m of the CD.
When the CD is used for recording and/or reproducing in the DVD optical system, the wavefront aberration is increased, and the shape of optical spot which can record and/or reproduce information, can not be obtained. It will be further detailed referring to the drawings. FIG. 16 is a view showing the relationship between the thickness of transparent substrate and the wavefront aberration. When the thickness of the transparent substrate is changed in an objective lens system, which is optimized under the condition that NA is 0.6, the wavelength of the laser beam emitted from the laser source is 635 nm, the thickness of the transparent substrate is 0.6 mm, and the refractive index of the transparent substrate is 1.58, the wavefront aberration is increased by 0.01.lambda. rms per 0.01 mm deviation of the thickness of the substrate. When the thickness of the transparent substrate deviates by .+-.0.07 mm, the wavefront aberration deviates by 0.07 .lambda.rms, and reaches the Marechal limit value which is a criterion within which recording and/or reproducing operations can normally be performed. As described above, when the thickness of the transparent substrate is switched to that (1.2 mm) of the CD in the optimally designed optical system in the thickness (0.6 mm) of the transparent substrate of the DVD, very large spherical aberration is generated, and thereby, it is difficult to record and/or reproduce both the CD and DVD by a single optical system.
Accordingly, as an optical pick-up apparatus to record and/or reproduce information of these 2 optical information recording medium, an optical pick-up apparatus using the optical system shown in FIG. 17 is widely known. In the optical system of the optical pick-up apparatus, an optical element which is a part of the optical system, is switched so that both the CD and DVD can be used. Referring to the drawing, it will be more precisely detailed. FIG. 17 is a sectional view of a recording and/or reproducing optical system for the optical information recording medium (CD, DVD) of a conventional example. In the drawing, when the DVD is recorded and/or reproduced, the light flux emitted from a laser source 11 passes through a hologram beam splitter 62, enters a collimator lens 21, and becomes parallel flux; this flux is limited to predetermined flux by an aperture-stop 51, passes through an objective lens 451, and a non-aberration light spot is image-formed on an information recording surface 91 through a transparent substrate 81 of an optical information recording medium 71. The light flux, modulated by an information pit and reflected by the information recording surface 91, returns to the hologram beam splitter 62 through the objective lens 451 and the collimator lens 21, separated here from an optical path from the laser source 11, and enters a light beam detector 61. This light beam detector 61 is composed of multi-divided PIN photodiodes, from each element of which a current proportional to the intensity of the entered light flux is outputted, and the current is sent to a detection circuit system, not shown in the drawing, in which an information signal, a focus error signal, and a track error signal are generated. According to these focus error signal and the track error signal, the objective lens 451 is controlled in the focusing direction and the tracking direction by a two-dimensional actuator (not shown in the drawing) composed of a magnetic circuit, a coil, etc., so that the light spot position is always adjusted onto the information track. When the CD is recorded or reproduced, the aperture-stop and the objective lens are respectively switched to an aperture-stop 52 and an objective lens 452. The light flux emitted from the laser source 11, enters a collimator lens 21, and becomes parallel light flux; the light flux is limited to the predetermined light flux by the aperture-stop 52, passes through the objective lens 452, and the non-aberration light spot is image-formed onto an information recording surface 92 through a transparent substrate 82 of an optical information recording medium 72.
However, in the optical pick-up apparatus, it is necessary to switch a portion of the optical system, and therefore, there are problems of troublesome operations, complicated structures, or an increase of the overall size of the apparatus.
Accordingly, a proposal has been made in which the DVD and CD can be recorded and/or reproduced by a single objective lens in the optical system of the optical pick-up apparatus.
According to a method disclosed in Japanese Patent Publication Open to Public Inspection No. 98431/1995, a hologram is provided on the information pick-up apparatus, and when the 0-order light and the 1-order light passing through the hologram, are respectively allocated to the DVD and CD, a spot, having diffraction limited performance corresponding to each optical information recording medium, can be obtained.
Further, according to a method disclosed in Japanese Patent Publication Open to Public Inspection No. 302437/1995, when a refractive surface of the objective lens is divided into a plurality of ring-shaped areas, a spot having diffraction limited performance corresponding to either of optical information recording media, can be obtained, although the objective lens is positioned at the almost same position in the direction of the optical axis with respect to both the DVD and the CD.
However, in the method in which the light flux from the laser source is divided for the DVD and the CD, in the methods disclosed in Japanese Patent Publication Open to Public Inspection Nos. 98431/1995 and 302437/1995, because plurality of light flux are always emitted to the information recording surface of the optical information recording medium, when information is read out by the light spot of one light flux, the other light flux does not contribute to the reading, and becomes unnecessary light, resulting in a factor of an increase of noise. Further, because the laser light intensity is divided for use, there are problems of a decrease of S/N ratio due to a decrease of a quantity of light, or an increase of power consumption and a decrease of the laser life when a quantity of light is increased.
As a problem of a technology by which the first optical information recording medium (DVD) which has a thin transparent substrate and has high density, and the second optical information recording medium (CD) which has a thick transparent substrate and has low density, can be recorded and/or reproduced without switching the optical system in the optical system of the above optical pick-up apparatus, a technology to obtain a more accurate light spot by which the first optical information recording medium and the second optical information recording medium can be recorded and/or reproduced, on respective information recording surfaces, is greatly required.